Feedback Control Loop for Bit Detection in an N-Dimensional Data Block

ABSTRACT

On existing DVD and CD players a control loop is required for the adaptation and timing recovery. For Two-Dimensional Optical Storage such a control loop has drawbacks because PRML detection in the form of a stripe-wise Viterbi detector is used. Such a detector introduces an increasing detection delay when going from the outer rows towards the center of the broad spiral. A feedback loop is arranged to determining an error signal from a first area of the data block where the first area is that area where the error signal can be determined within the shortest period of time. This reduces the duration of the detection step and thus increases the stability of the control loop.

This invention relates to a feedback control loop for controllingparameters of a signal comprised in a block of data stored in aN-dimensional data block on a record carrier where the decision directedfeedback loop comprises an input for receiving an information from therecord carrier and error signal derivation means for deriving an errorsignal from the information, a method for controlling parameters in afeedback control loop of a signal comprised in a block of data stored ina N-dimensional data block on a record carrier and an apparatus forreading an optical record carrier comprising a feedback control loop.

Such control loops are known from existing DVD and CD players where thecontrol loop is required for the adaptation and timing recovery.

For Two-Dimensional Optical Storage such a control loop has drawbacksbecause PRML detection in the form of a stripe-wise Viterbi detector isused. This detector introduces an increasing detection delay when goingfrom the outer rows towards the center of the broad spiral. For fastcontrol loops in a decision directed mode instability would occurbecause of the delay introduced by the detector. Further more the largespan of the 2D inter-symbol interference at higher densities and tilt,leads to a large 2D Equalizer which introduces even more delay. Inaddition, write-channel imperfections such as time-varying latticedistortion due to multiple-pass mastering require independent timingrecovery on each row within the broad spiral.

It is an objective of the invention to provide a stable control loop foruse in two dimensional optical storage.

To achieve this objective the control loop is characterized in that thefeedback loop is arranged to determine an error signal from a first areaof the N-dimensional data block where the first area is that area wherethe error signal can be determined within the shortest period of time.

Because the detection of data in areas, for instance rows, of a twodimensional data block, which can be for instance a section of a broadspiral, is performed in a predetermined order it is advantageous toselect that area of the data block where detection is performed firstand establish the error signal for the control loop based on thedetection results of this area. The stability of the control loop isconsequently increased when the delay caused by the detection step isminimized.

An embodiment of the feedback control loop is characterized in that thefirst area is a guard band area corresponding to the N-dimensional datablock.

The guard band comprises known data and consequently it is advantageousfor the detection to start from the guard band because the detectionneeds to deal with less unknown factors. By selecting the guard band asthe first area, i.e. the area where the error signal for the controlloop is derived from, a synergistic effect is achieved. The detection ismore reliable and the control loop is at the same time more stablecompared to another choice of starting point for the detection.

An embodiment of the feedback control loop is characterized in thatfeedback control loop is arranged for controlling parameters of a signalfrom a second area based on the error signal derived from the firstarea.

Instead of deriving an error signal from each area of the N-dimensionaldata block the error signal derived from the detection performed on thefirst area is also used for other areas of the N-dimensional data block.This greatly simplifies the control loop and ensures stability for theentire N-dimensional data block because the stability of the controlloop is only dependent on the delay introduced by the detectionperformed on the first area and not on the delay introduced by thedetection performed on the other areas.

An embodiment of the feedback control loop is characterized in that thesecond area is the N-dimensional data block.

Instead of deriving an error signal from each area of the N-dimensionaldata block only the error signal derived from the detection performed onthe first area is also used for all other areas of the N-dimensionaldata block. This greatly simplifies the control loop and ensuresstability for the entire N-dimensional data block because the stabilityof the control loop is only dependent on the delay introduced by thedetection performed on the first area and not on the delay introduced bythe detection performed on any of the other areas.

The basic assumption that leads to a solution is that the fast parametervariations are common for all rows within a broad spiral. Thisassumption is based on the insight of the physical mechanisms that leadto these variations in the channel. For instance, small variations inthe physical thickness of the cover layer of the disc (on top of theinformation layer containing the marks) can cause time-dependent channelvariations that are common to all bit-rows in the spiral i.e. it willgenerate some amount of spherical aberration in the read-out spot whichis common for all the bit-rows. This assumption allows the control loopsto do control on all rows based on information from the outer rows onlywhich have only a relative small detection delay.

An embodiment of the feedback control loop is characterized in that theparameters of the signal from the second area are uniformly controlledusing the error signal.

An embodiment of the feedback control loop is characterized in thatfeedback control loop is arranged for controlling parameters of a signalfrom a second area based on the error signal derived from the first areaand a further error signal derived from a third area. An N-dimensionaldata block of ten has more than one guard band. For instance atwo-dimensional data block in the form of a broad spiral can have twoguard bands, one guard band on each side of the broad spiral. Detectioncan start simultaneously from each guard band and progresssimultaneously towards each other in the direction of the center of theN-dimensional data block. There is consequently an equal delayintroduced by each detection performed on the various guard bands. Fromeach detection an error signal can be derived in an equal amount of timebut with differences in the actual error signal.

By taking multiple error signals into consideration no additional delayis introduced but a more appropriate response of the control loop isachieved.

An embodiment of the feedback control loop is characterized in that theparameters of the signal from the second area are uniformly controlledusing an average of the error signal and the further error signal.

When considering multiple error signals an average of the multiple errorsignals is an appropriate input for the control loop and allows a singleerror signal to be used for controlling the parameters of the signalfrom the second area in a uniform manner because deviations from theoptimum control of the parameters are minimized on average.

An embodiment of the feedback control loop is characterized in that theparameters of the signal from the second area are controlled using aninterpolated error signal derived by interpolating between the errorsignal and the further error signal based on a position of the thirdarea relative to the first area and the second area.

In reality small and probably slow variations occur relative betweenrows. These slow variations are combatted by correction loops that arebased on delayed information from the inner rows. These correction loopscan be controlled using an estimation of the appropriate error signalfor that inner row derived by interpolation between the error signalsderived from the guard bands.

An N-dimensional data block often has more than one guard band. Forinstance a two-dimensional data block in the form of a broad spiral canhave two guard bands, one guard band on each side of the broad spiral.Detection can start simultaneously from each guard band and progresssimultaneously towards each other in the direction of the center of theN-dimensional data block. There is consequently an equal delayintroduced by each detection performed on the various guard bands. Fromeach detection an error signal can be derived in an equal amount of timebut with differences in the actual error signal.

By taking multiple error signals into consideration no additional delayis introduced but a more appropriate response of the control loop isachieved.

The error signals derived from the detection performed on the two guardbands represent extremes in the data block and the appropriate errorsignal for the areas between the guard bands can be derived byinterpolation of the extremes. For instance when having a broad spiralwith 12 rows, of which row 1 and 12 are guard bands, the appropriateerror signal for row 4 can be determined by interpolation to be theerror signal of row 1 plus 40% of the differences between the errorsignal derived from row 1 and the error signal derived from row 12. Thisinterpolation is much quicker than waiting for the detection beingperformed on row 4 and thus increases the stability of the control loopwhen performing detection on row 4.

An embodiment of the feedback control loop is characterized in that thefeedback control loop comprises a detector with an input for receivingthe information from the input and an output for providing the errorsignal to the feedback control loop.

The detection can be integrated into the control loop or the controlloop can be directly coupled to the detection. In both cases the controlloop receives the error signal from a detector that has performeddetection on the information

An embodiment of the feedback control loop is characterized in that thefeedback control loop is a decision directed feedback control loop.

In a decision directed feedback control loop the delays introduced isquite large and this type of control loop benefits in particular fromthe application of the invention.

An embodiment of the feedback control loop is characterized in that afurther control loop, supplementing the control loop, is arranged todetermine an error signal from a fourth area of the N-dimensional datablock where the fourth area is different from the first area.

Often two control loops are used in cooperation with differentcharacteristics regarding stability. By supplying the error signalderived from the first area where delay is lowest that control loop isstabilized. The other cooperating control loop can be supplied with anerror signal from another area.

A method for controlling parameters in a feedback control loop accordingto the invention is characterized in that the method comprises the stepsof:

-   -   receiving an information from the record carrier    -   deriving an error signal from the information    -   determining an error signal from a first area of the        N-dimensional data block where the first area is that area where        the error signal can be determined within the shortest period of        time    -   controlling parameters based on the determined error signal.

Because the detection of data in areas, for instance rows, of a twodimensional data block, which can be for instance a section of a broadspiral, is performed in a predetermined order it is advantageous toselect that area of the data block where detection is performed firstand establish the error signal for the control loop based on thedetection results of this area. The stability of the control loop isconsequently increased when the delay caused by the detection step isminimized.

An embodiment of the method for controlling parameters in a feedbackcontrol loop is characterized in that the step of deriving an errorsignal from the information comprises the step of selecting theinformation from a guard band area corresponding to the N-dimensionaldata block

An embodiment of the method for controlling parameters in a feedbackcontrol loop is characterized in that the step of controlling parametersbased on the determined error signal comprises controlling parameters ofa signal from a second area based on the error signal derived from thefirst area.

An embodiment of the method for controlling parameters in a feedbackcontrol loop is characterized in that the second area is theN-dimensional data block.

An embodiment of the method for controlling parameters in a feedbackcontrol loop is characterized in that the parameters of the signal fromthe second area are uniformly controlled using the error signal.

An embodiment of the method for controlling parameters in a feedbackcontrol loop is characterized in that the step of controlling parametersbased on the determined error signal comprises controlling parameters ofa signal from a second area based on the error signal derived from thefirst area and a further error signal derived from a third area.

An embodiment of the method for controlling parameters in a feedbackcontrol loop is characterized in that the second area is theN-dimensional data block.

An embodiment of the method for controlling parameters in a feedbackcontrol loop is characterized in that the parameters of the signal fromthe second area are uniformly controlled using an average of the errorsignal and the further error signal.

An embodiment of the method for controlling parameters in a feedbackcontrol loop is characterized in that the step of controlling theparameters of the signal from the second area comprises the steps of:

-   -   interpolating between the error signal and the further error        signal based on a position of the third area relative to the        first area and the second area to derive an interpolated error        signal.

An embodiment of the method for controlling parameters in a feedbackcontrol loop is characterized in that the step of deriving an errorsignal from the information comprises the step of detecting symbols fromthe information and providing the error signal to the feedback controlloop.

An embodiment of the method for controlling parameters in a feedbackcontrol loop is characterized in that the feedback control loop is adecision directed feedback control loop.

An Apparatus for reading an optical record carrier comprising a feedbackcontrol loop benefits from the control loop according to the inventionbecause results in a stable control loop as required for the controllingof parameters of signals derived from the optical record carrier inparticular those with data stored in an N-dimensional data block such asa broad spiral.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Principle of Viterbi Detection.

FIG. 2: First possible organization scheme of the stripe-wise Viterbialong the broad spiral.

FIG. 3: Second, more advantageous organization scheme of the stripe-wiseViterbi.

FIG. 4: Block Diagram of the current receiver for record carriers withdata stored in a 2-dimensional pattern.

FIG. 5: Block Diagram of the timing recovery loop.

FIG. 6: Block Diagram for DC control based on the outer rows only.

FIG. 7: Block Diagram for DC control correction based on the inner rows.

FIG. 8: Block diagram in the Laplace domain of the fast control loop incombination with the slower correction loop.

FIG. 9: Variation in relative delay between adjacent rows in the broadspiral.

FIG. 10: Block diagram of the inner-outer control loop configuration forthe case of timing recovery (i.e. a 2nd order loop).

FIG. 11: Step responses of the coupled first order loops.

FIG. 12: Interpolation of the error signals for intermediate rows.

In Two-Dimensional Optical Storage bits are stored on a hexagonallattice. In contrast to conventional optical recording (CD, DVD, BD)where the bits are stored in a single spiral the bits in 2-dimensionalstorage are organized in so-called broad spirals. Each broad spiralconsist of a number of bit rows. Practical numbers are 9 and 11. Thebroad spirals are separated by a guard band consisting of a bit-rowwithout any pits (i.e. all zeros). This guard band introduces adiscontinuity in the phase relation between adjacent broad spirals toallow a constant areal density across the disc. Additionally, it servesas a starting point for 2D-bit detection. This bit-detection ispreferably done with a Viterbi detector. To reduce the enormouscomplexity of a full-fledged 2D Viterbi, the 2D Viterbi is divided intosmaller Viterbi detectors, each covering a limited number of bit-rowswhich are called stripes and have a typical width of 2 or 3 rows. Thisconfiguration is called a stripe-wise Viterbi Detector. The firstViterbi starts on the outer rows and uses the fact that the guard bandonly contains zeros as side information for the calculation of thereference levels in the branch metric calculation. The detected bits ofthis first Viterbi are passed to the next Viterbi to be used also hereas side information for the calculation of the reference levels. Thisprocedure is repeated until the last Viterbi processes the last rows ofthe broad spiral. FIG. 1 shows how it works for a single stripe 3. TheViterbi is going from state

Σ_(m) 1 to a next state σ_(n2). The branch metric is calculated as a sumof three contributions, one for each row within the stripe:

$\beta = {\sum\limits_{l = 1}^{h}\; {{{HF}_{i} - {REF}_{i,d}}}^{2}}$

To calculate the reference signal REF (or equivalently to determine theentry point in a look-up table that stores the REF values) the clustertype is needed. Further it can be assumed for the sake of simplicitythat the cluster type is based on the central bit 4 and the nearestneighbors 5A, 5B, 5C, 5E, 5F only (this is called the first shell only).To calculate the reference level for the central bit 4 the nearestneighbors 5A, 5B, 5C, 5E, 5F are needed as indicated by the hexagonal‘spider-web’ 6 in FIG. 1. Two of these nearest neighbors 5A, 5B are bitson the ‘outside bit row’ 7A i.e. are not part of the stripe 3 to beprocessed.

In a first concatenation scheme of these stripe Viterbi detectors V0,V1, V2, V3, V4, V5, V6 the blocks are organized linearly along the rowsin the broad spiral 21 as indicated in FIG. 2. Note that each viterbidetector V0, V1, V2, V3, V4, V5, V6 uses at the top row side informationobtained from the previous Viterbi (or from the guard band in case ofthe first Viterbi V0) and that at the bottom zeros are used as sideinformation because the bits are not yet knows. This causes the top-mostbit of the detected output of the Viterbi detectors V0, V1, V2, V3, V4,V5, V6 to be the most reliable one. Therefore, only this bit is used asoutput.

A second, more advantageous organization of the Viterbi detectors is tolayout the blocks in a ‘V’-shape. This is shown schematically in FIG. 3.Note, that in a final implementation two iterations are needed toachieve the required, low bit-error rate at the output of the viterbidetector V0, V1, V2, V3, V4, V5, V6, V7, V8, V9.

At the input of the bit-detector V0, V1, V2, V3, V4, V5, V6, V7, V8, V9bit-synchronous samples that are conform with a certain, so-calledtarget response are expected. This target response is the ideal responsedesirable for our optical channel. The same target response is used inthe Viterbi detector to calculate the reference levels. So ideally, theinput HF samples (see Eq. 1) are equal to the reference levels REF forthe correct cluster under evaluation and the branch metric is equal tozero. In reality however, the channel output signal is subject toimperfections (or a target response is chosen that does not fit thenominal channel perfectly; instead other criteria for the choice oftarget response such as white noise at the input of the detector can bechosen). These imperfections can be a gain mismatch, DC mismatch, timingerror, amplitude/phase distortion. Note further, that theseimperfections in the signal may be time varying. With signal processingin the form of gain/DC control loops, timing recovery, adaptiveequalizers, etc these imperfections (or mismatch) are eliminated as muchas possible.

A block diagram of a typical receiver is shown in FIG. 4.

The control loops in the adaptations block 41 need some input signalsthat indicate a mismatch between the actual signal at the input 42 ofthe bit-detector 43 and the ideally expected target signal. Therefore,the actual input signal of the bit-detector 43 is compared to thisideally expected target signal (by subtracting the signals herebygenerating a so-called error-signal). To calculate the ideally expectedtarget signal the bits are needed as they are stored on the medium. Insome cases these bits are known beforehand like in the case of apreamble. In this case a Data Aided (DA) mode is used where the targetsignal is calculated based on a known data file. However, in most casesthe bits on the disc are not known and an alternative must be found.

A solution to this problem is to use the bits that are detected by thebit-detector 43 although this bit-stream might contain some errors. Inthis case a Decision Directed (DD) mode is used. As an example thetiming recovery loop in the receiver 40 of FIG. 4 is shown in FIG. 5.Here the output of the bit detector 43 is used as input to a jitterdetector g_(k52) which calculates the ideal target signal dk. The actualsignal at the input of the detector y_(k) is compared with this signalby subtraction in subtractor 52. This results in an error signal e_(k).This error signal is mapped on a so-called signature signal bycorrelation (in the form of a sample-by-sample multiplication). Theresult is a timing error Δk which is input to the rest of the controlloop. From this structure it becomes clear that the bit-detector 43 isin the control loop and therefore the delay of the bit-detector becomesimportant.

In case of fast varying parameters high bandwidth control loops areneeded. There control loops allow only limited delay in the total loop.If the delay becomes larger the phase margin of these loops is reducedand stability problems occur. In particular for the receiver 40 of FIG.4 this is a big problem since the stripe-wise Viterbi configurationresults in large delay in the detector. For the outer rows the delay islimited to the back-tracking delay of the Viterbi . Note that this isbest case. In a practical implementation at least this delay is neededto build up the trellis and in addition extra delay might be introducedin the back-tracking algorithm. With tricks like ‘register exchange’however this delay might be minimized. For the inner rows however,detection cannot start until the side-information from the previousdetector is available. Therefore, the delay increases linearly startingfrom the outer rows going towards the center of the broad spiral wherethe last Viterbi block produces the output for three bit-rowssimultaneously unlike the other Viterbi blocks which only produce oneoutput bit-row. Typical delays can be larger than 100 bits. For thisreason the configuration of FIG. 3 is more beneficial than theconfiguration of FIG. 2 because the largest row delay in the viterbi ofFIG. 2 is twice as large as in the first configuration.

The basic assumption that leads to a solution is that the fast parametervariations are common for all rows within a broad spiral. Thisassumption is based on the insight of the physical mechanisms that leadto these variations in the channel. For instance, small variations inthe physical thickness of the cover layer of the disc (on top of theinformation layer containing the marks) can cause time-dependent channelvariations that are common to all bit-rows in the spiral (i.e. it willgenerate some amount of spherical aberration in the read-out spot whichis common for all the bit-rows). This assumption allows the controlloops to do control on all rows based on information from the outer rowsonly which have only a relative small detection delay. In realityhowever, small and probably slow variations occur relative between rows.These slow variations are combatted by correction loops that are basedon delayed information from the inner rows.

A first block diagram of this idea is shown in FIG. 6 for the case ofDC-compensation. First the errors of the outer rows e[k, 0] and e[k,N−1] are averaged by averaging means 61A, 61B to form a common errorsignal. This common error signal is multiplied by multiplier 62A by aproportional loop constant dc_fast. The loop filter is a simpleintegrator 63A and outputs a dc common signal that is used for all therows. The difference between the error signal of the outer rows is usedto compensate the outer rows for relative differences that might bepresent between these two rows. This is done by dividing the differencein error signal by 2 by the subtractor/divider 61B and use a secondproportional loop constant dc_slow which is multiplied with the resultof the subtractor/divider 61B, The result of this multiplication isinput to a simple integrator 63B. The output of the integrator 63A thesignal dc_diff is added to the DC signal by the adder 64A for thetop-row and subtracted by subtractor 64B from the DC-signal for thebottom row.

On top of this scheme correction loops are added that are based ondelayed information from the inner rows. This is shown in FIG. 7. Theerror signal from each of the inner loops is used as input to thecontrol loop for the corresponding row. Because both loops are firstorder loops it is expect that also the combination of both loops wouldshow first order behavior (i.e. an exponential convergence to the finalvalue in case of a step variation at the input). It appears however,that this is not true. To explain this behavior the combined controldiagram in the Laplace domain can be drawn as shown in FIG. 8. It can beshown that the transfer function from the input setpoint to the outputgain value for the outer rows is equal to:

$\begin{matrix}{G_{0} = {\frac{S_{0}}{S_{g}} = {\frac{K_{c}}{s + K_{c}}.}}} & (2)\end{matrix}$

which is a first order function leading to a step-response equal to:

s ₀ ^((t)=1) −e ^(K.t)  (3)

For the inner rows the transfer function is equal to:

$G_{i} = {\frac{S_{i}}{S_{g}} = \frac{{\left( {K_{c} + K_{i}} \right)s} + {K_{i}K_{c}}}{\left( {s + K_{i}} \right)\left( {s + K_{c}} \right)}}$

Leading to a step response equal to:

${g_{i}(t)} = {1 - {\frac{K_{i}}{K_{i} - K_{c}}^{- K_{i}^{t}}} + {\frac{K_{c}}{K_{i} - K_{c}}^{- K_{c}^{t}}}}$

A special case occurs when Ki=Kc=K. In that case the step response canbe calculated as:

The damping for the inner loops is equal to:

$\zeta = {\frac{1}{2}\left( {\sqrt{\frac{K_{i}}{K_{c}}} + \sqrt{\frac{K_{c}}{K_{i\;}}}} \right)}$

showing that the minimum damping factor is 1 for K_(i)=K_(c). This meansthat the system is always stable and thus can be used for our purpose.In practical situations K_(c) is much larger than K_(i) and the dampingfactor is even higher.

The same solution is applied to the timing recovery loops. However,generally the timing recovery loop is a second order loop. One of theintegrators is the numerically controlled oscillator. For recordcarriers with data stored in a 2-dimensional pattern a second ordertiming recovery loop is applied for all rows based on information fromthe outer rows only. This works perfectly under the assumption that allrows have exactly the same frequency and that the relative phase betweenthe rows does not vary. However, in practice a time-varying phasebetween adjacent rows in the same broad spiral is present due to themultiple-pass mastering that is currently applied to master the readonly record carriers with data stored in a 2-dimensional pattern withlaser beam recorders or electron beam recorders. To compensate for thistime-varying phase a first order phase correction loop is applied to theinner rows. Necessarily this loop is slow due to the large delays in thebit-detection for these rows. This is not a problem because it appearedthat also phase variation between rows is slow as shown in FIG. 9.

A block diagram of the second-order system is shown in FIG. 10. It canbe shown that the total transfer function for the inner rows can bewritten as:

$G_{i} = {\frac{K_{il}}{s + K_{il}} + \frac{\left( {K_{p}^{s} + K_{i}} \right)s}{\left( {s^{2} + {K_{p}s} + K_{i}} \right)\left( {s + K_{il}} \right)}}$

which can be shown also to be stable as long as the second order systemis stable.

A MatLab simulation of the step response based on the equations in theprevious section result in the graph of FIG. 11. The plot shows the stepresponse together with some results from processing of data comingretrieved by the playback device from record carriers with data storedin a 2-dimensional pattern. The first curve 110 shows the situation forthe inner correction loop when K₀ equals 4 times K_(i), the second curve111 shows the situation for the inner correction loop when Ki equals KO.The third loop 112

In FIG. 12 of this document shows that the common parameter that wastaken for the inner rows was derived from the parameter extracted fromthe outer rows by simply averaging the parameters for the outer rows andapplying this parameter for all the inner rows. This is schematicallyshown in the left part of the figure below for the gain parameter as anexample. However, it can be imagined that the gain value near row 0(e.g. row 1,2) is higher than the gain value near row N (e.g. row N-2,N-1) because the extracted gain value of row 0 was considerably higherthan row N. Therefore, it is reasonable to expect that a betterapproximation to the gain of the inner rows is obtained by linearinterpolation of the two extracted gains on the outer rows (as indicatedin the right figure below) instead of a simple averaging operation onthe two extracted gains on the outer rows.

1. A feedback control loop for controlling parameters of a signalcomprised in a block of data stored in a N-dimensional data block on arecord carrier where the feedback loop comprises an input for receivingan information from the record carrier and error signal derivation meansfor deriving an error signal from the information, characterized in thatthe feedback loop is arranged to determine an error signal from a firstarea of the N-dimensional data block where the first area is that areawhere the error signal can be determined within the shortest period oftime.
 2. A feedback control loop as claimed in claim 1, characterized inthat the control loop is a high bandwidth control loop.
 3. A feedbackcontrol loop as claimed in claim 1, characterized in that the first areais a guard band area corresponding to the N-dimensional data block
 4. Afeedback control loop as claimed in claim 1, characterized in that thefeedback control loop is arranged for controlling parameters of a signalfrom a second area based on the error signal derived from the firstarea.
 5. A feedback control loop as claimed in claim 4, characterizedthat a the feedback control loop is additionally arranged forcontrolling parameters of a signal from the second area based on anerror signal derived from the second area.
 6. A feedback control loop asclaimed in claim 4, characterized in that the second area is theN-dimensional data block.
 7. A feedback control loop as claimed in claim6, characterized in that the parameters of the signal from the secondarea are uniformly controlled using the error signal.
 8. A feedbackcontrol loop as claimed in claim 4, characterized in that feedbackcontrol loop is arranged for controlling parameters of a signal from asecond area based on the error signal derived from the first area and afurther error signal derived from a third area.
 9. A feedback controlloop as claimed in claim 8, characterized in that the second area is theN-dimensional data block.
 10. A feedback control loop as claimed inclaim 8, characterized in that the parameters of the signal from thesecond area are uniformly controlled using an average of the errorsignal and the further error signal.
 11. A feedback control loop asclaimed in claim 8, characterized in that the parameters of the signalfrom the second area are controlled using an interpolated error signalderived by interpolating between the error signal and the further errorsignal based on a position of the second area relative to the first areaand the third area.
 12. A feedback control loop as claimed in claim 1,characterized in that the feedback control loop comprises a detectorwith an input for receiving the information from the input and an outputfor providing the error signal to the feedback control loop.
 13. Afeedback control loop as claimed in claim 12, characterized in that thefeedback control loop is a decision directed feedback control loop. 14.A feedback control loop as claimed in claim 12, characterized in that afurther control loop, supplementing the control loop, is arranged todetermine an error signal from a fourth area of the N-dimensional datablock where the fourth area is different from the first area.
 15. Amethod for controlling parameters in a feedback control loop of a signalcomprised in a block of data stored in a N-dimensional data block on arecord carrier comprising the steps of receiving an information from therecord carrier deriving an error signal from the information determiningan error signal from a first area of the N-dimensional data block wherethe first area is that area where the error signal can be determinedwithin the shortest period of time controlling parameters based on thedetermined error signal
 16. A method for controlling parameters in afeedback control loop as claimed in claim 15, characterized in that thestep of deriving an error signal from the information comprises the stepof selecting the information from a guard band area corresponding to theN-dimensional data block
 17. A method for controlling parameters in afeedback control loop as claimed in claim 14, characterized in that thestep of controlling parameters based on the determined error signalcomprises controlling parameters of a signal from a second area based onthe error signal derived from the first area.
 18. A method forcontrolling parameters in a feedback control loop as claimed in claim17, characterized in that the second area is the N-dimensional datablock.
 19. A method for controlling parameters in a feedback controlloop as claimed in claim 18, characterized in that the parameters of thesignal from the second area are uniformly controlled using the errorsignal.
 20. A method for controlling parameters in a feedback controlloop as claimed in claim 16, characterized in that the step ofcontrolling parameters based on the determined error signal comprisescontrolling parameters of a signal from a second area based on the errorsignal derived from the first area and a further error signal derivedfrom a third area.
 21. A method for controlling parameters in a feedbackcontrol loop as claimed in claim 20, characterized in that the secondarea is the N-dimensional data block.
 22. A method for controllingparameters in a feedback control loop as claimed in claim 20,characterized in that the parameters of the signal from the second areaare uniformly controlled using an average of the error signal and thefurther error signal.
 23. A method for controlling parameters in afeedback control loop as claimed in claim 19, characterized in that thestep of controlling the parameters of the signal from the second areacomprises the steps of: interpolating between the error signal and thefurther error signal based on a position of the second area relative tothe first area and the third area to derive an interpolated errorsignal.
 24. A method for controlling parameters in a feedback controlloop as claimed in claim 1, characterized in that the step of derivingan error signal from the information comprises the step of detectingsymbols from the information and providing the error signal to thefeedback control loop.
 25. A method for controlling parameters in afeedback control loop as claimed in claim 23, characterized in that thefeedback control loop is a decision directed feedback control loop. 26.Apparatus for reading an optical record carrier comprising a feedbackcontrol loop as claimed in claim 1.